Search google for "firefox black squares bug" and a lot of hits come up. Example https://support.mozilla.org/en-US/kb...ons+and+themes

A lot of these issues seem to be people with HD78xx video cards, is that what you have? Are you running the latest video card drivers and latest version of Firefox etc.?

Try Firefox in safe mode? https://support.mozilla.org/en-US/kb...sing-safe-mode Maybe there's an extension doing something bad. You could always try re-seating your video card. Honestly I'm just randomly guessing ideas now.